An important detail to be aware of when using counties with public use microdata, is that counties are not directly identifiable in these data in samples from 1950 and onward. This is due to confidentiality restrictions placed on public use microdata that aim to protect the identity of respondents in the data. In IPUMS USA, however, some counties are identifiable because their boundaries overlap with other low-level geographical areas that are identified in the data. Therefore, only a subset of households actually live in a county that is identifiable. More information about this issue is available here and in this blog post. A listing of which counties are identifiable in which samples, from 1950 onward, is available here.

A couple alternatives are available. First, if you are looking to use microdata, you can choose other geographic variables available in IPUMS USA (see a list of these variables here). Second, if you are simply looking for aggregated statistics at the county level, the data available via IPUMS NHGIS may be useful. The data available via IPUMS NHGIS is provided in aggregated tables and is therefore not subject to the confidentiality restrictions placed on public use microdata.